Waveguide flange
Encyclopedia
A waveguide flange is a connector for joining sections of waveguide
, and is essentially the same as a pipe flange—a waveguide, in the context of this article, being a hollow metal conduit for microwave
energy. The connecting face of the flange is either square, circular or (particularly for large or reduce-height rectangular waveguides), rectangular. The connection between a pair of flanges is usually made with four or more bolts, though alternative mechanisms, such as a threaded collar, may be used where there is a need for rapid assembly and disassembly. Dowel pins are sometimes used in addition to bolts, to ensure accurate alignment, particularly for very small waveguides.
Key features of a waveguide join are; whether or not it is air-tight, allowing the waveguide to be pressurized, and whether it is a contact or a choke connection. This leads to three sorts of flange for each size of rectangular waveguide.
For rectangular waveguides there exist a number of competing standard flanges which are not entirely mutually compatible. Standard flange designs also exist for double-ridge, reduced-height, square and circular waveguides.
its breakdown voltage
and so increase the power that may be carried by the guide. This requires that all joins in the waveguide be airtight, something which is usually achieved by means of a rubber O-ring
recessed into a groove in the face of at least one of flanges forming each join. Gasket, gasket/cover or pressurizable flanges (such as that on the right of figure 2), are identifiable by the single circular groove which accommodates the O-ring. It is only necessary for one of the flanges in each pressurizable connection to be of this type; the other may have a plain flat face (like that in figure 1). This ungrooved type is known as a cover, plain or unpressurizable flange.
It is also possible to form air-tight seal between a pair of otherwise unpressurizable flanges using a flat gasket
made out of a special electrically conductive elastomer. Two plain cover flanges may be mated without such a gasket, but the connection is then not pressurizable.
of the waveguides, and must cross the join between them if microwave power is to pass through the connection without reflection or loss.
of the current across the crack will cause further damage, loss of power, and may give rise to arcing from one side of the guide to the other, thereby short circuiting
it.
In the absence of unpressurizable choke flanges, all flanges fall into one of three categories: choke, gasket/cover and cover.
An E-plane
cross section of an assembled choke connection is shown in figure 3. This is the plane cutting each of the broad walls of the waveguide along its centre-line, which is where the longitudinal surface currents—those that must cross the join—are at their strongest. The choke ditch and the gap between the flange faces together form a somewhat convoluted side-branch to the path of the main guide. This side branch is designed to present a low input impedance
where it meets the broad walls of the waveguide, so that the surface currents there are not obstructed by the gap, but instead flow onto and off of the separated faces of the flanges. Conversely, on the outer edge of the choke ditch, at the point where the two flanges come into physical contact, the ditch presents a high series impedance. The current through the contact point is thus reduced to a small value, and the danger of arcing across any crack between the flanges is likewise reduced.
the electric field also varies in going around the ditch, having two changes of polarity, or one complete wave in the circumference. The ditch thus constitutes a quarter-wave resonant short-circuit stub, and has a high (ideally infinite) input impedance at its mouth. This high impedance is in series with the metal-to-metal connection between the flanges, and minimizes the current across it. The distance from the main waveguide through the gap to the ditch is likewise one quarter of a wavelength in the E-plane. The gap thus forms a quarter-wave transformer
, transforming the high impedance at the top of the ditch to a low (ideally zero) impedance at the broad wall of the waveguide.
(that is roughly from 1.3 to 1.9 time the guide cutoff).
with the assistance of Shep Roberts while the two were working at the MIT Radiation Lab
during World War II. Winfield Salisbury also claims to have made the invention while leader of the Radio Frequency Group at the MIT Radiation Lab between 1941 and 1942. The invention was not patented.
of -46 dB) over a useful bandwidth, and eliminate the danger of arcing at the join. Nevertheless, better performance is possible with a carefully made contact-connection between undamaged plain flanges.
or brazed
together, the end of the tube is machined down so that it is perfectly level with the face. This type of construction can be seen in figures 1, 4 and 5.
ed to form a socket
which fits onto the end of the waveguide tubing. The two pieces are soldered or brazed together to ensure an uninterrupted conducting path between the inside surface of the waveguide tube and the mouth of the flange. This type of construction can be seen in figure 2, and is shown diagramatically in figure 3. A variation on this is butt-mounting, in which the waveguide tube abuts the back face of the flange. The back of the flange has a number of protrusions, sufficient to align the tube, but without forming an unbroken socket-wall around it.
Socket mounting avoids the need to machine the face of the flange during attachment. For choke flanges this means that the depth to which the face is recessed, and the width of the resulting gap is fixed when the flange is manufactured and will not change when it is attached. MIL-spec choke flanges are socket-mounted.
waveguide, and formerly
also for single-ridge guide.
MIL-Spec flanges have designations of the form UG-xxxx/U where the xs represent a variable-length catalogue number, not in itself containing any information about the flange.
These standards are works of the U.S. government, and are freely available online from the U.S. Defense Logistics Agency
.http://www.dscc.dla.mil/Programs/MilSpec/DocSearch.asp
(IEC) standard IEC 60154 describes flanges for square
and circular waveguides,
as well as for what it refers to as flat,
medium-flat,
and ordinary
rectangular guides.
IEC flanges are identified by an alphanumeric code consisting of; the letter U, P or C for Unpressurizable (plain cover), Pressurizable (with a gasket groove) and Choke (also with a gasket groove); a second letter, indicating the shape and other details of the flange and finally the IEC identifier for the waveguide. For standard rectangular waveguide the second letter is A to E, where A and C are round flanges, B is square and D and E are rectangular. So for example UBR220 is a square plain cover flange for R220 waveguide (that is, for WG20, WR42), PDR84 is a rectangular gasket flange for R84 waveguide (WG15, WR112) and CAR70 is a round choke flange for R70 waveguide (WG14, WR137).
The IEC standard is endorsed by a number of European standards organizations, such as the British Standards Institution
.
(EIA) is the body that defined the WR designations for standard rectangular waveguides. EIA flanges are designated CMR (for Connector, Miniature, Rectangular waveguide) or CPR (Connector, Pressurizable, Rectangular waveguide) followed by the EIA number (WR number) for the relevant waveguide. So for example, CPR112 is a gasket flange for waveguide WR112 (WG15).
Waveguide (electromagnetism)
In electromagnetics and communications engineering, the term waveguide may refer to any linear structure that conveys electromagnetic waves between its endpoints. However, the original and most common meaning is a hollow metal pipe used to carry radio waves...
, and is essentially the same as a pipe flange—a waveguide, in the context of this article, being a hollow metal conduit for microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
energy. The connecting face of the flange is either square, circular or (particularly for large or reduce-height rectangular waveguides), rectangular. The connection between a pair of flanges is usually made with four or more bolts, though alternative mechanisms, such as a threaded collar, may be used where there is a need for rapid assembly and disassembly. Dowel pins are sometimes used in addition to bolts, to ensure accurate alignment, particularly for very small waveguides.
Key features of a waveguide join are; whether or not it is air-tight, allowing the waveguide to be pressurized, and whether it is a contact or a choke connection. This leads to three sorts of flange for each size of rectangular waveguide.
For rectangular waveguides there exist a number of competing standard flanges which are not entirely mutually compatible. Standard flange designs also exist for double-ridge, reduced-height, square and circular waveguides.
Pressurization
The atmosphere within waveguide assemblies is often pressurized to raisePaschen's law
Paschen's Law, named after Friedrich Paschen, was first stated in 1889. He studied the breakdown voltage of gas between parallel plates as a function of pressure and gap distance. The voltage necessary to arc across the gap decreased up to a point as the pressure was reduced. It then increased,...
its breakdown voltage
Breakdown voltage
The breakdown voltage of an insulator is the minimum voltage that causes a portion of an insulator to become electrically conductive.The breakdown voltage of a diode is the minimum reverse voltage to make the diode conduct in reverse...
and so increase the power that may be carried by the guide. This requires that all joins in the waveguide be airtight, something which is usually achieved by means of a rubber O-ring
O-ring
An O-ring, also known as a packing, or a toric joint, is a mechanical gasket in the shape of a torus; it is a loop of elastomer with a disc-shaped cross-section, designed to be seated in a groove and compressed during assembly between two or more parts, creating a seal at the interface.The O-ring...
recessed into a groove in the face of at least one of flanges forming each join. Gasket, gasket/cover or pressurizable flanges (such as that on the right of figure 2), are identifiable by the single circular groove which accommodates the O-ring. It is only necessary for one of the flanges in each pressurizable connection to be of this type; the other may have a plain flat face (like that in figure 1). This ungrooved type is known as a cover, plain or unpressurizable flange.
It is also possible to form air-tight seal between a pair of otherwise unpressurizable flanges using a flat gasket
Gasket
thumb|sright|250px|Some seals and gaskets1. [[o-ring]]2. fiber [[Washer |washer]]3. paper gaskets4. [[cylinder head]] [[head gasket|gasket]]...
made out of a special electrically conductive elastomer. Two plain cover flanges may be mated without such a gasket, but the connection is then not pressurizable.
Electrical continuity
Electric current flows on the inside surfaceSkin effect
Skin effect is the tendency of an alternating electric current to distribute itself within a conductor with the current density being largest near the surface of the conductor, decreasing at greater depths. In other words, the electric current flows mainly at the "skin" of the conductor, at an...
of the waveguides, and must cross the join between them if microwave power is to pass through the connection without reflection or loss.
Contact connection
A contact connection is formed by the union of any combination of gasket and cover flanges, and ideally creates a continuous inner surface from one waveguide to the other, with no crack at the join to interrupt the surface currents. The difficulty with this sort of connection is that any manufacturing imperfections or dirt or damage on the faces of the flanges will result in a crack. ArcingElectric arc
An electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. An arc discharge is characterized by a lower voltage than a glow discharge, and relies on...
of the current across the crack will cause further damage, loss of power, and may give rise to arcing from one side of the guide to the other, thereby short circuiting
Short circuit
A short circuit in an electrical circuit that allows a current to travel along an unintended path, often where essentially no electrical impedance is encountered....
it.
Choke connection
A choke connection is formed by mating one choke flange and one cover (or gasket/cover) flange (but never two choke flanges). The central region of the choke flange face is very slightly recessed so that it does not touch the face of the cover flange, but is separated from it by a narrow gap. The recessed region is bounded by a deep choke trench (or ditch or groove) cut into the face of the flange. Choke flanges are only used with rectangular waveguide, and are invariably pressurizable, having a gasket groove encircling the choke ditch. The presence of these two concentric circular grooves makes choke flanges easily recognizable. The left-hand flange in figure 2 is a choke flange.In the absence of unpressurizable choke flanges, all flanges fall into one of three categories: choke, gasket/cover and cover.
An E-plane
E-plane and H-plane
The E-plane and H-plane are reference planes for linearly polarized waveguides, antennas and other microwave devices.- E-Plane :For a linearly-polarized antenna, this is the plane containing the electric field vector and the direction of maximum radiation. The electric field or "E" plane determines...
cross section of an assembled choke connection is shown in figure 3. This is the plane cutting each of the broad walls of the waveguide along its centre-line, which is where the longitudinal surface currents—those that must cross the join—are at their strongest. The choke ditch and the gap between the flange faces together form a somewhat convoluted side-branch to the path of the main guide. This side branch is designed to present a low input impedance
Input impedance
The input impedance of an electrical network is the equivalent impedance "seen" by a power source connected to that network. If the source provides known voltage and current, such impedance can be calculated using Ohm's Law...
where it meets the broad walls of the waveguide, so that the surface currents there are not obstructed by the gap, but instead flow onto and off of the separated faces of the flanges. Conversely, on the outer edge of the choke ditch, at the point where the two flanges come into physical contact, the ditch presents a high series impedance. The current through the contact point is thus reduced to a small value, and the danger of arcing across any crack between the flanges is likewise reduced.
Theory
At the operational frequency of the choke flange, the depth of the ditch is approximately one quarter of a wavelength. This is somewhat longer than a quarter of the free-space wavelength, sinceHelmholtz equation
The Helmholtz equation, named for Hermann von Helmholtz, is the elliptic partial differential equation\nabla^2 A + k^2 A = 0where ∇2 is the Laplacian, k is the wavenumber, and A is the amplitude.-Motivation and uses:...
the electric field also varies in going around the ditch, having two changes of polarity, or one complete wave in the circumference. The ditch thus constitutes a quarter-wave resonant short-circuit stub, and has a high (ideally infinite) input impedance at its mouth. This high impedance is in series with the metal-to-metal connection between the flanges, and minimizes the current across it. The distance from the main waveguide through the gap to the ditch is likewise one quarter of a wavelength in the E-plane. The gap thus forms a quarter-wave transformer
Quarter wave impedance transformer
A quarter-wave impedance transformer, often written as λ/4 impedance transformer, is a component used in electrical engineering consisting of a length of transmission line or waveguide exactly one-quarter of a wavelength long and terminated in some known impedance. The device presents at its...
, transforming the high impedance at the top of the ditch to a low (ideally zero) impedance at the broad wall of the waveguide.
Frequency dependence
Because the working of a choke connection depends on the wavelength, its impedance can be zero at at most one frequency within the operating band of the waveguide. However, by making the gap extremely narrow, and the choke ditch relatively wide, the input impedance can be kept small over a broad frequency band. For gap and ditch widths in a fixed proportion, the connection input impedance is approximately proportional to either width (doubling both widths is like having two connections in series). Increasing just the ditch width, increases its input impedance proportionately, and to a some extent decreases the transformed impedance, though the effect is limited when the gap-length is not exactly one quarter wavelength. The MIL-spec choke flanges have a gap width of between 2% and 3% of the waveguide height (the smaller inner dimension of the guide), which for WR28 waveguide (WG22) amounts to a gap of just 3 thousandths of an inch. The choke ditch in these flanges is some 8 times wider (around 20% of the waveguide height), although the proportions vary considerably, as the width-to-height ratio of the standard mid-size guides deviates from 2:1. MIL-Spec choke flanges are intended for use over the full recommended operational frequency band of the waveguide(that is roughly from 1.3 to 1.9 time the guide cutoff).
History
Claimants to the invention of the choke connection include Norman RamseyNorman Foster Ramsey, Jr.
Norman Foster Ramsey, Jr. was an American physicist. A physics professor at Harvard University since 1947, Ramsey also held several posts with such government and international agencies as NATO and the United States Atomic Energy Commission...
with the assistance of Shep Roberts while the two were working at the MIT Radiation Lab
Radiation Laboratory
The Radiation Laboratory, commonly called the Rad Lab, was located at the Massachusetts Institute of Technology in Cambridge, Massachusetts and functioned from October 1940 until December 31, 1945...
during World War II. Winfield Salisbury also claims to have made the invention while leader of the Radio Frequency Group at the MIT Radiation Lab between 1941 and 1942. The invention was not patented.
Performance
Choke connections can achieve a VSWR of 1.01 (a returnReturn loss
In telecommunications, return loss or reflection loss is the loss of signal power resulting from the reflection caused at a discontinuity in a transmission line or optical fiber. This discontinuity can be a mismatch with the terminating load or with a device inserted in the line...
of -46 dB) over a useful bandwidth, and eliminate the danger of arcing at the join. Nevertheless, better performance is possible with a carefully made contact-connection between undamaged plain flanges.
Attachment to waveguide
Flanges are either through-mounted or socket-mounted on the end of the waveguide tube.Through-mounting
In through-mounting, the waveguide tube passes all the way through to the front face of the flange. Initially the tube is allowed to protrude slightly beyond the face of the flange, then after the two pieces have been solderedSoldering
Soldering is a process in which two or more metal items are joined together by melting and flowing a filler metal into the joint, the filler metal having a lower melting point than the workpiece...
or brazed
Brazing
Brazing is a metal-joining process whereby a filler metal is heated above and distributed between two or more close-fitting parts by capillary action. The filler metal is brought slightly above its melting temperature while protected by a suitable atmosphere, usually a flux...
together, the end of the tube is machined down so that it is perfectly level with the face. This type of construction can be seen in figures 1, 4 and 5.
Socket-mounting
In socket-mounting, the aperture in the front face of the flange matches the inside dimensions of the waveguide. At the back, the aperture is rabbetRabbet
A rabbet is a recess or groove cut into the edge of a piece of machineable material, usually wood. When viewed in cross-section, a rabbet is two-sided and open to the edge or end of the surface into which it is cut....
ed to form a socket
Socket
Socket may refer to:In mechanics:* Socket wrench, a type of wrench that uses separate, removable sockets to fit different sizes of nuts and bolts...
which fits onto the end of the waveguide tubing. The two pieces are soldered or brazed together to ensure an uninterrupted conducting path between the inside surface of the waveguide tube and the mouth of the flange. This type of construction can be seen in figure 2, and is shown diagramatically in figure 3. A variation on this is butt-mounting, in which the waveguide tube abuts the back face of the flange. The back of the flange has a number of protrusions, sufficient to align the tube, but without forming an unbroken socket-wall around it.
Socket mounting avoids the need to machine the face of the flange during attachment. For choke flanges this means that the depth to which the face is recessed, and the width of the resulting gap is fixed when the flange is manufactured and will not change when it is attached. MIL-spec choke flanges are socket-mounted.
Standards
MIL-Spec
http://www.dscc.dla.mil/Programs/MilSpec/ListDocs.asp?BasicDoc=MIL-DTL-3922 is a United States Military Standard giving detailed descriptions of choke, gasket/cover and cover flanges for rectangular waveguide. http://www.dscc.dla.mil/Programs/MilSpec/ListDocs.asp?BasicDoc=MIL-DTL-39000 describes flanges for double-ridgewaveguide, and formerly
also for single-ridge guide.
MIL-Spec flanges have designations of the form UG-xxxx/U where the xs represent a variable-length catalogue number, not in itself containing any information about the flange.
These standards are works of the U.S. government, and are freely available online from the U.S. Defense Logistics Agency
Defense Logistics Agency
The Defense Logistics Agency is an agency in the United States Department of Defense, with more than 26,000 civilian and military personnel throughout the world...
.http://www.dscc.dla.mil/Programs/MilSpec/DocSearch.asp
IEC
International Electrotechnical CommissionInternational Electrotechnical Commission
The International Electrotechnical Commission is a non-profit, non-governmental international standards organization that prepares and publishes International Standards for all electrical, electronic and related technologies – collectively known as "electrotechnology"...
(IEC) standard IEC 60154 describes flanges for square
and circular waveguides,
as well as for what it refers to as flat,
medium-flat,
and ordinary
rectangular guides.
IEC flanges are identified by an alphanumeric code consisting of; the letter U, P or C for Unpressurizable (plain cover), Pressurizable (with a gasket groove) and Choke (also with a gasket groove); a second letter, indicating the shape and other details of the flange and finally the IEC identifier for the waveguide. For standard rectangular waveguide the second letter is A to E, where A and C are round flanges, B is square and D and E are rectangular. So for example UBR220 is a square plain cover flange for R220 waveguide (that is, for WG20, WR42), PDR84 is a rectangular gasket flange for R84 waveguide (WG15, WR112) and CAR70 is a round choke flange for R70 waveguide (WG14, WR137).
The IEC standard is endorsed by a number of European standards organizations, such as the British Standards Institution
BSI Group
BSI Group, also known in its home market as the British Standards Institution , is a multinational business services provider whose principal activity is the production of standards and the supply of standards-related services.- History :...
.
EIA
The Electronic Industries AllianceElectronic Industries Alliance
The Electronic Industries Alliance was a standards and trade organization composed as an alliance of trade associations for electronics manufacturers in the United States. They developed standards to ensure the equipment of different manufacturers was compatible and interchangeable...
(EIA) is the body that defined the WR designations for standard rectangular waveguides. EIA flanges are designated CMR (for Connector, Miniature, Rectangular waveguide) or CPR (Connector, Pressurizable, Rectangular waveguide) followed by the EIA number (WR number) for the relevant waveguide. So for example, CPR112 is a gasket flange for waveguide WR112 (WG15).